Assessing the interaction between history of usage and plant invasions: Bamboo as a case study
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Studies in invasion science often focus on the biological or environmental implications of invasive alien species. However, biological invasions are inherently due to the human-mediated dispersal of species; this means that there needs to be a greater focus on the socio-economic context of why species have been introduced by humans, and how humans have created environments or situations to foster the success of alien taxa. This dissertation explores the interaction of human usage and plant invasions using bamboos as a case study, with the intention of answering the following questions: Why are some species selected by humans over others? How does this influence invasion success, impacts, and the cultural role of alien species in receiving environments? I found that, like many other alien taxa, the transfer of bamboos globally has been non-random. Large-statured Asiatic species and those with a greater number of cultivars are more likely to have been introduced outside of their native ranges. The species with high introduction rates are the bamboos that are found to be invasive; this supports the notion that propagule pressure is a universally important factor in facilitating invasions. However, in contrast to many other studies, biogeographic status (the native status) of bamboo was not a strong predictor of the type and magnitude of impacts. Impacts are rather explained by certain human activities (disturbance such as logging and clearing, increasing temperatures related to climate change, and the promotion of bamboo monocultures in mixed forests for commercial purposes). As such, the management of highly competitive native species should be considered in conjunction with the management of invasive alien species in forest ecosystems where bamboos are present. The competitive nature of bamboos that leads to impacts can be applied to other tall-statured grasses. Specifically, rapid growth rate and the capacity to accumulate biomass (a function of height) allow many tall grasses to form monospecific stands, accumulate dense and deep litter mats, reduce light availability and alter fire and nutrient-cycling regime. Naturalisation rates are greater in tall-statured grasses compared to other grasses, and the pathways of introduction are generalizable (e.g. for addressing environmental problems). Tall-statured grasses are a useful functional group for predicting high-risk taxa and for making generalised management plans. Looking at South Africa as a case-study, I found 26 alien species of bamboo recorded as introduced, with populations of several species widely naturalised around the country. I also found bamboos to be an inherently difficult group to identify to the species level, emphasizing the caution that must be taken with regard to future introductions. Amongst the public there is a complex tapestry of perceptions towards bamboos related to (1) a long history of introduction, and multiple introduction events, where bamboos have become culturally significant for some groups of people, and (2) the realisation that they can cause problems. In conclusion, many bamboos can be classified as synanthropic species in that they have benefited from human activity or the habitats that humans create around them. Studying bamboos has provided further insights into how social and economic imperatives are shaping a new biota at a global level.